Make char::DecodeUtf16::size_hist more precise
New implementation takes into account contents of `self.buf` and rounds lower bound up instead of down.
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1 changed files with 15 additions and 3 deletions
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@ -120,9 +120,21 @@ impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
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#[inline]
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fn size_hint(&self) -> (usize, Option<usize>) {
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let (low, high) = self.iter.size_hint();
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// we could be entirely valid surrogates (2 elements per
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// char), or entirely non-surrogates (1 element per char)
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(low / 2, high)
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// `self.buf` will never contain the first part of a surrogate,
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// so the presence of `buf == Some(...)` always means +1
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// on lower and upper bound.
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let addition_from_buf = self.buf.is_some() as usize;
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// `self.iter` could contain entirely valid surrogates (2 elements per
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// char), or entirely non-surrogates (1 element per char).
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//
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// On odd lower bound, at least one element must stay unpaired
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// (with other elements from `self.iter`), so we round up.
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let low = low.div_ceil(2) + addition_from_buf;
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let high = high.and_then(|h| h.checked_add(addition_from_buf));
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(low, high)
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}
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}
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